Studies were made of the morphology of the kidney and heart in 10-month old, stroke-prone spontaneously hypertensive rats (SHRSP) which had been maintained for nine months on one of three experimental diets: standard (STD; 25.3% crude protein and 0.5% methionine), low protein (LP: 19% crude protein and 0.39% methionine), or high methionine (MET: 25.3% crude protein and 2.0% methionine). Except for 24 and 40 weeks of age, SHRSP/LP had significantly lower blood pressure than those of SHRSP/STD and SHRSP/LP. Throughout the experiment, normotensive Wistar Kyoto rats fed standard diet (WKY/STD) maintained significantly lower BP than the SHRSP groups. From 4 to 10 months of age, SHRSP/STD and SHRSP/LP (25% - 63% and 46% - 80%, respectively) exhibited abnormal behavior which included excitement, repetitive lifting, hyperirritability and coarse tremor of the paws. At 10 months of age, the degree of cardiac hypertrophy and fibrosis, thickening of intramyocardial arterioles and the presence of inflammatory cells were similar among the three experimental SHRSP groups. The kidneys of SHRSP/LP demonstrated marked thickening of the vascular walls, narrowing of the lumina of arterioles, and glomerular sclerosis. Since the morphology of the kidneys from SHRSP/MET differed markedly from that of SHRSP/STD but appeared close to that of SKY/STD, it seems from these results that increasing the amount of methionine in the diet provides protection against the renal complications of hypertension in SHRSP; however, further investigation is required to determine whether this protective action is primarily a result of dietary effect on blood pressure or dietary influence on some metabolic changes that slow the rate and degree of tissue damage in prolonged hypertension.